The present invention relates generally to quality control testing of printed circuit boards and especially to preparation of samples to facilitate testing of the quality of printed circuit boards with metallized holes. During hole plating of printed boards, levelness and thickness of the metal plating are most important.
Up to now, the only certain way to be able to make an acceptable estimation is to cut the holes through a center plane. Then, the metal thickness is measured using a microscope. Thus, it is very important that the measuring be made at a plane as near the center plane of a metal plated hole as possible. The error from the center plane to a radius which intersects the measuring plane must not exceed 5 degrees, i.e. at a hole diameter of 1 mm about 50-60 .mu. and at 0.3 mm about 10-320 .mu..
One of the disadvantages with this method is that it requires considerable amounts of plastic, of which the major part has to be ground away which is both time consuming and expensive. Testing according to this technique is mentioned in Danish patent No. 125 217.
A considerably better way of practising this principle is described in the Swedish patent application No. 8301136-1. Here, the embedded samples are milled to approximately the center of the holes to be tested. Thereafter, material is milled on both sides of the holes so that a narrow raised part is formed which then can be ground for investigation.
This means considerable savings both in time and material compared with the method of the Danish patent.
In principle, this method can be exact enough for holes with a diameter of about 1 mm or larger, but when the hole diameter is 0.3 mmm or smaller, the deviations are too large. Besides, IEC, IPC and US military rules stipulate that three holes in a row are to be tested. The tolerances when drilling printed boards may be around .+-.50 .mu.. It might be nearly impossible to get three random holes in a row with a linear deviation of .+-.10-20 .mu..